Infected doubter dismissing Covid initially (6)

I believe the answer is:

septic

I'm a little stuck... Click here to teach me more about this clue!

'infected' is the definition.
(I know that infected can be written as septic)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for septic that I've seen before include "Contaminated with disease-causing bacteria" , "Poisoned" , "Caused by putrefaction" , "Putrid" , "Type of tank" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app